Elastic wave devices are widely used for manufacturing resonators, filters, duplexers and sensors operating at RF frequencies. Examples of typical elastic wave devices include surface acoustic wave (SAW) devices using the propagation of elastic waves at the surface of a piezoelectric surface, and bulk acoustic wave (BAW) devices using the propagation and resonances of elastic waves in a thickness of thin films. Packaging is an important part of the cost of these devices. To reduce costs, wafer level packaging technology where the package is manufactured collectively at the wafer level is typically employed.
Another example is disclosed in U.S. Pat. No. 7,310,027 to Kando for a Boundary Acoustic Wave Filter which employs a boundary acoustic wave or interface wave and uses the propagation of such waves at the interface of two materials. In this case, the transducers used to generate and detect the waves are interdigital transducers (IDT) similar to the transducers used for SAW devices. The main difference between this device and other elastic wave devices is that the IDTs are placed at the interface of two materials.
There is a need to make filters capable of working with unbalanced inputs and balanced outputs or balanced inputs and unbalanced outputs, as desired. Typically, one way to achieve this with SAW or BAW devices is to use a coupled resonator filter (CRF). These filters generally include several IDTs between reflective gratings. Such CRFs are also used to design single-ended filters, since they usually provide a desirable far-out rejection.
However, a problem associated with the CRF is its power handling. The number of electrodes of the transducer (IDT) has to be small enough to provide a wide bandwidth. Consequently, the energy density is large and the power handling is poor. In contrast for ladder filters, the size of the transducer is much larger resulting in a more desirable power handling.
By way of further example regarding techniques typically used by those of skill in the art, reference is made to H. Okitsu et al. for “A basic study of mode coupling SAW device with face to face bonding”, proc. of symposium on ultrasonics electronics, vol. 30(2009), pp 279-280, 18-20 Nov. 2009; K. Koh et al., for “Study of SAW device having face to face aligned package structure”, 2008 IEEE Ultrasonics symp. proc., pp 1596-1599; and Meister et al. for “Component That Operates Using Acoustic Waves and Method for Producing Said Component, US Patent Application Publication No. US 2008/94150 A1, the disclosures of which are herein incorporated by reference in their entirety.
Okitsu describes a way to manufacture SAW filters using face to face bonding, wherein two substrates with SAW devices are bonded to form a cavity. The two SAW devices are separated by a gap. One advantage of such a structure and method is that there is no need for a typical wafer package. There is some coupling between the two devices but since there is no material between the two substrates, the coupling is only electrostatic.
Koh discloses a similar device wherein a liquid with an elastic property is used to provide some acoustic coupling between two SAW devices. In both of the Okitsu and Koh references, bonding is done in a region far from the transducer. Acoustic coupling exists through a liquid.
Meister discloses a device using a piezoelectric film between a substrate and electrodes in an IDT on each side of the substrate. In one described embodiment, the acoustic excitation is done along a direction in a plane while the coupling is done through the piezoelectric film. Such an approach makes it difficult to achieve a desirable electro-acoustic coupling for an excitation by the IDT at the surface of a piezoelectric film. As described, the busbars of the transducers face each other through the piezoelectric layer. This is equivalent to having a BAW resonator between the input and the output of the device, and it will result in deterioration of the filter rejection and/or ripple in the passband. Further, the variation of the frequency versus the temperature may be large. A film of silicon oxide may reduce this variation. In addition, to obtain desirable results, the wave has to be guided in the layers. To ensure this, the velocity of the wave in the film has to be smaller than the velocity of the wave in the substrate.
There remains a need for a device and method for reducing package size for an acoustic wave device that provides for a single balanced operation. Desirably, such devices should use relatively large transducers, should have reduced temperature sensitivity, and should obtain low load and/or source impedances. The present invention satisfies such needs.